This invention relates to a crystal containing material.
The manufacture of diamond matrix composites or tool materials or components, such as saw segments, grinding wheels and polycrystalline diamond (PCD) products, is well established. There are various methods used in their manufacture. For saw segments, pellets, pearls and the like, diamond powder is mixed with matrix material and the mixture sintered, substantially at atmospheric pressure, to produce the component. Alternatively, the molten matrix material is infiltrated into a bed of the diamond powder, also substantially at atmospheric pressure, to produce the component. For PCD products, diamond powder is sintered under conditions of high temperature and high pressure in the presence of a solvent-catalyst, and the resultant piece shaped afterwards to produce the final component.
In essence, each method and product starts with diamond powder and then the component is fabricated. Some of these methods are conducted at substantially atmospheric pressure, and components produced by them, are restricted to matrices that sinter or infiltrate at relatively low temperature so that graphitisation of the diamond is avoided or minimised.
In the manufacture of PCD products, the solvent-catalyst is restricted usually to the matrices used for the production of the cemented lungsten carbide support, or in the case of the more thermally stable products, the infiltrant is restricted to those elements or compounds which react with the diamond to form a desirable phase, e.g. silicon forming silicon carbide.
According to the present invention, a method of making a bonded, coherent material comprising a mass of crystals in a matrix includes the steps of providing a source of the crystals of the type to be grown and which are substantially free of macroscopically faceted surfaces, producing a reaction mass by bringing the source crystals into contact with a suitable solvent/catalyst, subjecting the reaction mass to conditions of elevated temperature and pressure suitable for crystal growth in the reaction zone of a high temperature/high pressure apparatus to produce the material and removing the material from the reaction zone, the conditions of crystal growth being chosen such that the source crystals are converted to crystals having developed macroscopic facets of low Miller index.
Particles with a high proportion of high Miller index surfaces will yield faceted crystals more readily than particles with a low proportion of high Miller index faces. Further, particles with a low proportion of high Miller index surfaces may only facet partially and/or show dissolution facets. The bonded, coherent material will contain crystals in which a high proportion, generally at least 50% and preferably at least 70% of which, have developed facets of low Miller index. These crystals, particularly if they are diamond crystals, will generally be single crystals.
The method of generating the supersaturation driving force necessary for crystal growth used in the practice of this invention depends, at least in part, and preferably predominantly, upon the difference in surface free energy between low Miller index surfaces and higher Miller index surfaces which is hereinafter referred to as xe2x80x9cthe Wulff effectxe2x80x9d; higher Miller index surfaces having a higher surface free energy than lower Miller index surfaces. The equilibrium shape of a crystal occurs when the minimum total surface free energy per unit volume of crystal is attained, i.e. when the crystal is bounded by surfaces of low Miller index. Higher Miller index surfaces can be considered to comprise a series of stepped low Miller index surfaces in close proximity to one another. Such a situation is included in the term xe2x80x9chigher Miller index surfacexe2x80x9d. When a crystal is in its equilibrium shape, there exists a point whose perpendicular distance from every face is proportional to the surface free energy of that face. This is the basis of Wulff""s theorem.
It has been found that in the case of diamond, the preferred crystal in the practice of the invention, the difference in surface-free energy between high Miller index surfaces and low Miller index surfaces is large and can generate a supersaturation which sustains crystallisation when diamond crystals in various sizes, including those tens of microns in size, are used. Thus, the invention has particular application to the growth of diamond crystals wherein supersaturation is created, at least in part, and preferably predominantly, by a difference of solubility in crystal surfaces of high Miller index and crystal surfaces of lower Miller index, e.g. by the reduction of surface free energy by the substantial elimination of steps, kinks and other structural defects which characterise macroscopic high Miller index surfaces.
It has further been observed that the Wulff effect is dependent on the conditions which prevail in the reaction mass. For example, for a given solvent/catalyst and pressure applied, the Wulff effect is dependent on temperature and time, as can be seen from the graphs shown in FIGS. 1 and 2. The graph of FIG. 1 shows the temperature dependence of the Wulff effect on diamond in an iron-nickel solvent/catalyst at about 5,4 GPa, with this condition being maintained for one hour. The graph of FIG. 2 shows the temperature dependence of the Wulff effect on diamond in the same iron-nickel solvent/catalyst at about 5,4 GPa with the condition being maintained for ten hours. From these graphs, it will be noted that the larger the source crystal size the higher the applied temperature to ensure that the Wulff effect dominates and the production of a crystal mass containing a high proportion of single crystals having facets of low Miller index is achieved. Similar graphs can be produced for other solvent/catalysts and applied pressures to determine under what conditions the Wulff effect dominates.
The conditions of elevated temperature and pressure will vary according to the nature of the crystals. For diamond crystals the elevated temperature will generally be in the range 1100 to 1500xc2x0 C. and the elevated pressure generally in the range 4,5 to 7 GPa.
The bonded, coherent material made by the method of the invention may, for example, be a tool component or insert, bearing surface, substrate for further processing, abrasive material, heat sink biomedical material, catalyst body or the like. These materials all use the properties of the crystal the matrix or a combination of the properties of the crystal and matrix.
The material may have zones of different properties. For example, the zones may vary in crystal concentration or size, or in matrix or in a combination thereof. The differing zones may extend in layers or regions which are distributed in a random or ordered way, for example, from one side of the material to an opposite side or may extend in layers from a central point to an outside surface of the material.
The invention has particular application to materials which have a crystal content of less than 85 percent by volume, generally less than 60% by volume.
The material may be produced in such manner as to provide it with a support layer in the form of a substrate, an external surface layer or an internal core to which it is bonded. The nature of the support layer may be chosen to complement the properties or enhance the utility of the material. The interface between the material and the support layer may be of any shape including planar, convex, concave or irregular.